1. Field of the Invention
The present invention relates to an excavator, particularly to an excavator provided with arms which are telescopically stretchable in the longitudinal direction thereof and a bucket for digging and removing earth and sand. A modified excavator is provided with a hook at the tip end of the arm.
2. Description of Prior Art
There has been widely used an excavator in an area where earth working, such as digging trenches or holes, is carried out (hereinafter referred to as the working area), which excavator comprises a boom having a slightly C-shaped configuration and swingably mounted on a movable body, an arm connected to the tip end of the boom and having a substantially linear shape so as to be vertically movable relative to the boom and a bucket mounted at the tip end of the arm. An excavator called a back hoe has been used widely in road construction and for burying objects. The boom, the arm and the bucket cooperate so that the bucket is pushed into the earth to dig the earth and sand and is raised to remove the dug earth and sand. The fundamental arrangement of the excavator is principally the same, namely, it includes three hydraulic cylinders cooperating with each other for carrying out the digging operation.
However, the conventional excavator has the following drawbacks.
A first drawback is that the length of the boom and the length of the arm, respectively, need to be extended to deepen the trench and the hole. Furthermore, when the earth and sand are picked and collected by the excavator at a river side, inasmuch as the lengths of the boom and the arm are fixed, the bucket cannot reach beyond a predetermined length so that the depth of digging and the length to reach the earth and sand are limited. It is theoretically possible to extend the lengths of the boom and the arm to dig deeper or to permit the bucket to reach a longer distance. However, if the lengths of the bucket and the arm are extended, the excavator becomes long as a whole which entails difficult transportation. In this case, if the boom is positioned perpendicularly relative to the mobile body so as to turn the bucket after collecting the earth and sand, the length of the boom becomes so long that it is liable to contact an electric wire or a construction structure.
That is, the first drawback is that it was impossible to lengthen the distance that the bucket extends from the movable body since the lengths of the boom and the arm are fixed and the connecting portions between the movable body, the boom, the arm and the bucket are merely operated at joints thereof. Hence, when the earth and sand is to be dug deeply, a long boom is necessitated, which is very inconvenient.
A second drawback is as follows. The conventional excavator has a function to dig deeply, which is very effective for mechanically digging a large volume of earth and sand and is very improved in the working efficiency compared with manpower. However, at the working area digging working is usually accomplished by burying working for burying pipes, including a Hume concrete pipe, to return to the original position or covering working for covering the dug trench or the hole by an iron plate. There is no problem to raise or lower materials having a weight that can be handled by manpower. However, in the case of heavy materials, such as the Hume concrete pipe or the iron plate, there is required an exclusive raising and lowering means, such as a crane, from the safety point of view. The crane meets the requirement of a safe standard of working.
It has been very rare to employ a working step that requires a crane in the working area in addition to an excavator. Furthermore, there seldom occurs the case that two vehicles having different functions occupy the same working area. When the working area comprises a main working area and a neighboring area, such as narrow side roads surrounding the main working area from which the excavator enters, only the excavator enters the working area for thereby preventing the crane from entering the working area. Still furthermore, since the operating hours of the crane are so short compared with those of the excavator, there is caused idle time for the crane even it if occupies the working area.
In the case of raising the heavy material during the operation of digging the earth and sand, a wire is hung from the bucket of the excavator and the heavy material is suspended by the wire. Thereafter the boom supporting the bucket is vertically moved to raise the heavy material. Although the operation to raise the heavy material is very simple, there is a likelihood that the wire will slip from the bucket since the excavator has no function inherently to raise the heavy material. This use of the excavator, which is different from the inherent use, as a crane, is involved in a dangerous working operation since the weight limit of the material to be suspended by the bucket is unknown.
In view of the inconvenience of the excavator, the applicant proposed an excavator, as disclosed in Japanese Patent Application No. 63-315787, having a crane incorporated in an excavator. The excavator having the contractible crane mechanism which is provided at the side of the arm or accommodated inside the arm is so structured that the crane is stretched from the arm when raising the heavy material and a wire is hung down from the tip end of the crane mechanism, then a hook is hung down from the tip end of the wire. With this arrangement, when the heavy material is not raised, the crane mechanism is contracted so as not to obstruct the digging operation by the bucket. Hence, this excavator is very convenient to use in a narrow working area since it carries out two functions by a single unit, namely, the function to raise the material and the function to dig the earth and sand.
The proposed excavator having the crane function has, however, the drawback that the crane mechanism is to be contracted so as not to hinder the digging operation and this entails a complicated mechanism. Furthermore, the crane mechanism is separately provided in addition to the arm and the boom which requires many manufacturing steps and high cost.
To solve the first drawback, the applicant proposed an excavator provided with telescopically stretchable arms and a bucket attached to a distal end of the arms. One of the arms can be lowered to the deepest position in the working area or extended to a longest position in the working area as disclosed in Japanese Patent Application No. 1-107990. However, the hydraulic cylinder for controlling the angular distance or position of the bucket relative to the arm is moved simultaneously with the movement of the inner arm relative to the outer arm. Hence, the hydraulic cylinder is so designed that a part of the hydraulic cylinder is movable relative to the outer arm and a base of the hydraulic cylinder is moved by the front or the rear wire in synchronism with the movement of the inner arm. However, this proposed excavator has such a drawback that the base of the hydraulic cylinder is not movable in synchronism with the inner arm since the front and the rear wires are all the time stretched, which entails a complicated mechanism.